1. Technical Field
The invention relates generally to methods and procedures for maintaining well control during drilling operations. More specifically, the invention relates to methods and procedures where xe2x80x9criserlessxe2x80x9d drilling systems are used.
2. Background Art
Exploration companies are continually searching for methods to make deep water drilling commercially viable and more efficient. Conventional drilling techniques are not feasible in water depths of over several thousand feet. Deep water drilling produces unique challenges for drilling aspects such as well pressure control and wellbore stability.
Deep water drilling techniques have, in the past, typically relied on the use of a large diameter marine riser to connect drilling equipment on a floating vessel or a drilling platform to a blowout preventer stack on a subsea wellhead disposed on the seafloor. The primary functions of the marine riser are to guide a drill string and other tools from the floating vessel to the subsea wellhead and to conduct drilling mud and earth cuttings from a subsea well back to the floating vessel. In deeper waters, conventional marine riser technology encounters severe difficulties. For example, if a deep water marine riser is filled with drilling mud, the drilling mud in the riser may account for a majority of the drilling mud in the circulation system. As water depth increases, the drilling mud volume increases. The large volume of drilling mud requires an excessively large circulation system and drilling vessel. Moreover, an extended length riser may experience high loads from ocean currents and waves. The energy from the currents and waves may be transmitted to the drilling vessel and may damage both the riser and the vessel.
In order to overcome problems associated with deep water drilling, a technique known as xe2x80x9criserlessxe2x80x9d drilling has been developed. Not all riserless techniques operate without a marine riser. The marine riser may still be used for the purpose of guiding the drill string to the wellbore and for protecting the drill string and other lines that run to and from the wellbore. When marine risers are used, however, they typically are filled with seawater rather than drilling mud. The seawater has a density that may be substantially less than that of the drilling mud, substantially reducing the hydrostatic pressure in the drilling system.
An example of a riserless drilling system is shown in U.S. Pat. No. 4,813,495 issued to Leach and assigned to the assignee of the present invention. A riserless drilling system 10 of the ""495 patent is shown in FIG. 1 and comprises a drill string 12 including drill bit 20 and positive displacement mud motor 30. The drill string 12 is used to drill a wellbore 13. The system 10 also includes blowout preventer stack 40, upper stack package 60, mud return system 80, and drilling platform 90. As drilling is initiated, drilling mud is pumped down through the drill string 12 through drilling mud line 98 by a pump which forms a portion of mud processing unit 96. The drilling mud flow operates mud motor 30 and is forced through the bit 20. The drilling mud is forced up a wellbore annulus 13A and is then pumped to the surface through mud return system 80, mud return line 82, and subsea mudlift pump 81. This process differs from conventional drilling operations because the drilling mud is not forced upward to the surface through a marine riser annulus.
The blowout preventer stack 40 includes first and second pairs of ram preventers 42 and 44 and annular blowout preventer 46. The blowout preventers (xe2x80x9cBOPxe2x80x9ds) may be used to seal the wellbore 13 and prevent drilling mud from travelling up the annulus 13A. The ram preventers 42 and 44 include pairs of rams (not shown) that may seal around or shear the drill string 12 in order to seal the wellbore 13. The annular preventer 46 includes an annular elastomeric member that may be activated to sealingly engage the drill string 12 and seal the wellbore 13. The blowout preventer stack 40 also includes a choke/kill line 48 with an adjustable choke 50. The choke/kill line 48 provides a flow path for drilling mud and formation fluids to return to the drilling platform 90 when one or more of the BOPs (42, 44, and 46) have been closed.
The upper end of the BOP stack 40 may be connected to the upper stack package 60 as shown in FIG. 1. The upper stack package 60 may be a separate unit that is attached to the blowout preventer stack 40, or it may be the uppermost element of the blowout preventer stack 40. The upper stack package 60 includes a connecting point 62 to which mud return line 82 is connected. The upper stack package 60 may also include a rotating head 70. The rotating head 70 may be a subsea rotating diverter (xe2x80x9cSRDxe2x80x9d) that has an internal opening permitting passage of the drill string 12 through the SRD. The SRD forms a seal around the drill string 12 so that the drilling mud filled annulus 13A of the wellbore 13 is hydraulically separated from the seawater. The rotating head 70 typically includes both stationary elements that attach to the upper stack package 40 and rotating elements that sealingly engage and rotate with the drill string 12. There may be some slippage between rotating elements of the rotating head 70 and the drill string 12, but the hydraulic seal is maintained. During drill pipe xe2x80x9ctripsxe2x80x9d to change the bit 20, the rotating head 70 is typically tripped into the hole on the drill string 12 before fixedly and sealingly engaging the upper stack package 60 that is connected to the BOP stack 40.
The lower end of the BOP stack 40 may be connected to a casing string 41 that is connected to other elements (such as casing head flange 43 and template 47) that form part of a subsea wellhead assembly 99. The subsea wellhead assembly 99 is typically attached to conductor casing that may be cemented in the first portion of the wellbore 13 that is drilled in the seafloor 45. Other portions of the wellbore 13, including additional casing strings, well liners, and open hole sections extend below the conductor casing.
The mud return system 80 includes the subsea mudlift pump 81 that is positioned in the mud return line 82 adjacent to the upper stack package 60. The subsea mudlift pump 81 in the ""495 patent is shown as a centrifugal pump that is powered by a seawater driven turbine 83 that is, in turn, driven by a seawater transmitting powerfluid line 84. The mud return system 80 boosts the flow of drilling mud from the seafloor 45 to the drilling mud processing unit 96 located on the drilling platform 90. Drilling mud is then cleaned of cuttings and debris and recirculated through the drill string 12 through drilling mud line 98.
When drilling a well, particularly an oil or gas well, there exists the danger of drilling into a formation that contains fluids at pressures that are greater than the hydrostatic fluid pressure in the wellbore. When this occurs, the higher pressure formation fluids flow into the well and increase the fluid volume and fluid pressure in the wellbore. The influx of formation fluids may displace the drilling mud and cause the drilling mud to flow up the wellbore toward the surface. The formation fluid influx and the flow of drilling and formation fluids toward the surface is known as a xe2x80x9ckick.xe2x80x9d If the kick is not subsequently controlled, the result may be a xe2x80x9cblowoutxe2x80x9d in which the influx of formation fluids (which, for example, may be in the form of gas bubbles that expand near the surface because of the reduced hydrostatic pressure) blows the drill string out of the well or otherwise destroys a drilling apparatus. An important consideration in deep water drilling is controlling the influx of formation fluid from subsurface formations into the well to control kicks and prevent blowouts from occurring.
Drilling operations typically involve maintaining the hydrostatic pressure of the drilling mud column above the formation fluid pressure. This is typically done by selecting a specific drilling mud density and is typically referred to as xe2x80x9coverbalancedxe2x80x9d drilling. At the same time, however, the bottom hole pressure of the drilling mud column must be maintained below the formation fracture pressure. If the bottom hole pressure exceeds the formation fracture pressure, the formation may be damaged or destroyed and the well may collapse around the drill string.
A different type of drilling regime, known as xe2x80x9cunderbalancedxe2x80x9d drilling, may be used to optimize the rate of penetration (xe2x80x9cROPxe2x80x9d) and the efficiency of a drilling assembly. In underbalanced drilling, the hydrostatic pressure of the drilling mud column is typically maintained lower than the fluid pressure in the formation. Underbalanced drilling encourages the flow of formation fluids into the wellbore. As a result, underbalanced drilling operations must be closely monitored because formation fluids are more likely to enter the wellbore and induce a kick.
Once a kick is detected, the kick is typically controlled by xe2x80x9cshutting inxe2x80x9d the wellbore and xe2x80x9ccirculating outxe2x80x9d the formation fluids that entered the wellbore. Referring again to FIG. 1, a well is typically shut in by closing one or more BOPs (42, 44, and/or 46). The fluid influx is then circulated out through the adjustable choke 50 and the choke/kill line 48. The choke 50 is adjustable and may control the fluid pressure in the well by allowing a buildup of back pressure (caused by pumping drilling mud from the mud processing unit 96) so that the kick may be circulated through the drilling mud processing unit 96 in a controlled process. The drilling mud processing unit 96 has elements that may remove any formation fluids, including both liquids and gases, from the drilling mud. The drilling mud processing unit 96 then recirculates the xe2x80x9ccleanedxe2x80x9d drilling mud back through the drill string 12. Typically, as the kick is circulated out, the drilling mud that is being pumped back into the wellbore 13 through drill string 12 has an increased density of a preselected value. The resulting increased hydrostatic pressure of the drilling mud column may equal or exceed the formation pressure at the site of the kick so that further kicks are prevented. This process is referred to as xe2x80x9ckilling the well.xe2x80x9d The kick is circulated out of the wellbore and the drilling mud density is increased in substantially one complete circulation cycle (for example, by the time the last remnants of the drilling mud with the pre-kick mud density have been circulated out of the well, mud with the post-kick mud density has been circulated in as a substitute). When the wellbore is stabilized, drilling operations may be resumed or the drill string 12 may be tripped out of the wellbore 13. This method of controlling a kick is typically referred to as the xe2x80x9cWait and Weightxe2x80x9d method. The Wait and Weight Method has historically been the preferred method of circulating out a kick because it generally exerts less pressure on the wellbore 13 and the formation and requires less circulating time to remove the influx from the drilling mud.
Another method for controlling a kick is typically referred to as the xe2x80x9cDriller""s Method.xe2x80x9d Generally, the Driller""s Method is accomplished in two steps. First, the kick is circulated out of the wellbore 13 while maintaining the drilling mud at an original mud weight. This process typically takes one complete circulation of the drilling mud in the wellbore 13. Second, drilling mud with a higher mud weight is then pumped into the wellbore 13 to overcome the higher formation pressure that produced the kick. Therefore, the Driller""s Method may be referred to as a xe2x80x9ctwo circulation killxe2x80x9d because it typically requires at least two complete circulation cycles of the drilling mud in the wellbore 13 to complete the process.
A device known as a drill string valve (xe2x80x9cDSVxe2x80x9d) may be used as a component of either of the previously referenced well control methods. A DSV is typically located near a bottom hole assembly and includes a spring activated mechanism that is sensitive to the pressure inside the drill string. When drill string pressure is lowered below a preselected level, the spring activates a flow cone that moves to block flow ports in a flow tube. In order for drilling mud to flow through the drill string, the flow ports must be at least partially open. Thus, the DSV permits flow through the drill string if sufficient surface pump pressure is applied to the drilling fluid column, and the DSV typically only permits flow in one direction so that it act as a check valve against mud flowing back toward the surface.
The spring pressure in the DSV may be adjusted to account for factors such as the depth of the wellbore, the hydrostatic pressure exerted by the drilling mud column, the hydrostatic pressure exerted by the seawater from a drilling mud line to the surface, and the diameter of drill pipe in the drill string. The drilling mud line may be defined as a location in a well where a transition from seawater to drilling mud occurs. For example, in the system 10 shown in FIG. 1, the drilling mud line is defined by the hydraulic seal of the rotating head 70 that separates the drilling mud of the wellbore annulus 13A from seawater. The DSV may be used to stop drilling mud from experiencing xe2x80x9cfree-fallxe2x80x9d when the mud circulation pumps are shut down and the well is shut-in.
Using the system of the Leach ""495 patent as an example, when the pumps of the mud processing unit 96 are shut down and no DSV is present in the drill string 12, the mud column hydrostatic pressure in the drill string 12 is greater than the sum of the hydrostatic pressure of the drilling mud in the wellbore annulus 13A and a suction pressure generated by the subsea mudlift pump 81. Drilling mud, therefore, free-falls in the drill string into the wellbore annulus 13A until the hydrostatic pressure of the mud column in the drill string 12 is equalized with the sum of the hydrostatic pressure of the drilling mud in the wellbore annulus 13A and the mudlift pump 81 suction pressure. Thus, the well continues to flow while equilibrium is established. The continued flow of drilling mud in the well after pump shut-down may typically be referred to as an xe2x80x9cunbalanced U-tubexe2x80x9d effect. The DSV, which should be in a closed position after the pumps are shut-down, may prevent the free-fall of drilling mud in the wellbore that may be attributable to the unbalanced U-tube.
In contrast, in conventional drilling systems where drilling mud is returned to the surface through the wellbore annulus, the drilling mud circulation system forms a xe2x80x9cbalanced U-tubexe2x80x9d because there is no flow of drilling mud in the well after the surface pumps are shut down. The well does not flow because the hydrostatic pressure of the drilling mud in the drill string is balanced with the hydrostatic pressure of the mud in the wellbore annulus.
Well control procedures may be complicated by a leaking DSV. For example, the spring in the DSV must be adjusted correctly so that it will activate the flow cone and block the flow ports when pressure is removed from the mud column such as by shutting down the surface mud pumps. If the flow ports remain at least partially open, the well will continue to flow after all the pumps have been shut down and/or after the well has been fully shut-in. Further, the DSV may develop leaks from flow erosion, corrosion, or other factors.
Typically, there are two conditions where the DSV may be checked for leaks. The first condition is during normal drilling operations when, for example, circulation of drilling mud is stopped so that a drill pipe connection may be made (all pumps must be shut off for the DSV check). In this case, an effort is made to distinguish between a leaking DSV and a possible kick. The second condition occurs after the well has been fully shut-in on a kick (again, all pumps must be shut off for the DSV check). In this case, an effort is made to distinguish between a leaking DSV and additional flow that may have entered the well from the known kick. In both cases it is important to check the DSV for leaks because otherwise it may be difficult to determine if additional flow in the well is due to a leaking or partially open DSV or to additional flow that has entered the well from a kick.
Reliable methods are needed to quickly and efficiently control and eliminate kicks that are experienced when drilling wells. The methods must account for the special configurations of deepwater drilling systems and must function both with and without the use of a DSV. The methods must also be designed to determine the difference between a leaking DSV and a kick that may have occurred during drilling operations, and also between a leaking DSV and additional flow that may occur after a kick is shut-in. In either case, the kicks come from formations with pore pressures that exceed the fluid pressure in the wellbore. Finally, the methods should result in a hydrostatically xe2x80x9cdeadxe2x80x9d well so that the drill string may be removed from the wellbore or so that drilling operations may resume.
One aspect of the invention is a method for controlling a subsea well including shutting at least one blowout preventer, opening at least one isolation line, and circulating a formation fluid influx out of a well while an inlet pressure of a subsea mudlift pump is adjusted to maintain a substantially constant drill pipe pressure at an initial circulating pressure. Kill weight drilling mud is pumped from the surface into the well, and the drill pipe pressure is reduced according to a preselected drill pipe pressure decline schedule until the kill weight drilling mud reaches a bottom of the well. After the kill weight drilling mud reaches the bottom of the well, the drill pipe pressure is maintained at a final circulating pressure by adjusting the inlet pressure of the subsea mudlift pump. Kill weight drilling mud is then circulated from the bottom of the well to the surface at the final circulation pressure.
In another aspect, the invention is a method for controlling a subsea well including shutting at least one blowout preventer, opening at least one isolation line, and circulating a formation fluid influx out of a well while an inlet pressure of a subsea mudlift pump is adjusted to maintain a substantially constant drill pipe pressure at an initial circulating pressure. Kill weight drilling mud is pumped from the surface into the well, and the inlet pressure of the subsea mudlift pump is held substantially constant until the kill weight drilling mud reaches a bottom of the well. The inlet pressure of the subsea mudlift pump is then adjusted to maintain the drill pipe pressure at a final circulating pressure. Kill weight drilling mud is then circulated from the bottom of the well to the surface at the final circulating pressure.
Other aspects and advantages of the invention will be apparent from the following description and the appended claims.